a) Field of the Invention
The invention is directed to a method and arrangements for the suppression of debris in short-wavelength radiation sources based on a plasma in which the short-wavelength radiation that is emitted from the plasma and is in a vacuum chamber is directed through a debris filter with at least one mechanical filter structure before the short-wavelength radiation reaches the collector optics. It is preferably applied in EUV sources for semiconductor lithography.
Besides the desired EUV radiation, light sources for EUV radiation that are based on a radiation-emitting hot plasma also generate debris, as it is called. By debris is meant any form of particles that are undesirable within the meaning of the present application and that exit the plasma and negatively influence the performance or stability of the application components, particularly the life of optical surfaces.
Within the framework of EUV sources, the following types of debris can be distinguished:                a) fast, high-energy atomic particles/ions which damage optical surfaces through sputtering (energies are typically 1-10 keV);        b) slow particles which can deposit on optical surfaces (typical energies corresponding to a temperature of less than 5000 K or less than 1 eV);        c) large macroscopic particles from electrode material.        
In collector mirrors such as those used in microlithography for focusing the EUV radiation in an intermediate focus (multilayer mirrors or metallic mirrors with grazing light incidence), debris of type a) leads to a rapid loss of reflectivity through destruction of the sensitive coating. Since this effect is brought about by sputtering on the optical surface and the resulting removal of material, the decisive function of the arrangement according to the invention is either to eliminate the fast particles or to slow them down to kinetic energies significantly below 10 eV which no longer allow sputtering.
Failure to completely eliminate debris of type a) is due to the lack of availability of suitable materials which must be equally resistant to the considerable loading by both radiation and debris and which, on the other hand, must be highly transparent for the EUV radiation. Therefore, many different methods for reducing the particle flow have already been presented and discussed in a great many publications.
b) Description of the Related Art
For example, DE 102 15 469 B4 describes a debris filter in which means for generating an electrical field orthogonal to the central propagation direction of the divergent EUV beam bundle are arranged downstream of the outlet opening of the vacuum chamber and in which means are provided for generating a gas sink resulting in a particle flow parallel to the direction of the electrical field.
U.S. Pat. No. 6,359,969 B1 discloses a device containing a radiation source and a processing organ for processing the radiation of the radiation source. A filter containing a plurality of (planar) foils or plates oriented in radial direction from the punctiform radiation source is arranged between the radiation source and the processing organ.
U.S. Pat. No. 6,683,936 describes an EUV-transparent interface structure in the form of a membrane or channel structure for optically connecting a first and a second chamber while suppressing the contaminating particle flow of the source. Inside the chamber, an inert gas flow is directed sideways to or onto the source in order to keep the particles away from the application.
U.S. Pat. No. 6,566,668 B2 is directed to a high-energy photon source in which a pair of plasma pinch electrodes is arranged in a vacuum chamber. A debris collector between the pinch region and the radiation collector comprises a large number of passages, each of which is oriented toward the light beams, so that it passes the radiation and retards the debris which moves along more random paths. In addition, a flow of gas is directed toward the source location of the radiation in order to further slow the debris particles.
Further, DE 102 37 901 B3 describes a debris filter comprising a plurality of blades. The blades have the shape of essentially flat surfaces with a pair of parallel edges, are arranged so as to be uniformly distributed radially with respect to a given optical axis of the radiation source, and are arranged orthogonally between an inner enclosing surface and an outer enclosing surface which are arranged parallel to one another and rotationally symmetric so as to arch over the plasma for a preselected solid angle around the optical axis.
Similarly, the emission of debris from an x-ray source is prevented by a debris filter system according to U.S. Pat. No. 6,867,843 B2 in that an attraction unit has an attracting surface parallel to an axis passing through the emission point. Further, the debris filter has a rotation unit which rotates the attraction unit around said axis.
The known arrangements have the disadvantage that they are not equally effective for all types of particles or absorb too much EUV radiation. For example, neutral particles from the plasma are not influenced by electromagnetic fields. High field strengths combined with high voltages are necessary for influencing fast particles. Parasitic gas discharges may be caused by applying these voltages. Apart from increased shadowing effects, rotating blade filters have the disadvantage that very fast particles are only deflected corresponding to the rotating direction. The necessary high pressures are not achieved in gas flows or gas curtains, but a considerable proportion of the desired EUV radiation is also absorbed.